1. Field of the Invention
The present invention relates to a friction plate used in a clutch and/or a brake of an automatic transmission (AT) of a vehicle and a wet-type multi-plate clutch having such a friction plate. More specifically, the present invention relates to an improvement in an oil passage formed in a friction surface of a friction plate.
2. Description of the Related Art
In general, in a wet-type multi-plate clutch, friction plates and separator plates are arranged alternately between a drum and a hub of a clutch or a brake, and, engagement and disengagement of the clutch are performed by pressurizing and releasing a clutch piston.
Further, recently, request for reducing fuel consumption in a vehicle has been increased more and more, and thus, also in an automatic transmission, in order to reduce power loss during the disengagement of the clutch, further reduction of drag torque between the friction plate and the separator plate has been requested.
In general, in many cases, the wet-type multi-plate clutch used in the automatic transmission (AT) is designed so that lubricating oil can easily escape from an inner peripheral side to an outer peripheral side of the friction plate in order to reduce the power loss, thereby reducing the drag torque. Such means for reducing the drag torque are already known and are disclosed, for example, in Japanese Patent Application Laid-Open Nos. H11-141570 and 2005-76759. In clutches disclosed in the Japanese Patent Application Laid-Open Nos. H11-141570 and 2005-76759, a friction plate is provided with an oil groove having an closed end at an inner peripheral side and serving to separate friction plates and separator plates during the disengagement of the clutch, and an oil supplying oil passage extending through from an inner radial end to an outer radial end and serving to prevent seizure by supplying lubricating oil onto friction surfaces during the engagement of the clutch.
However, recently, in order to enhance response of the transmission for the purpose of achieving not only enhanced reduction of fuel consumption but also enhancement of power performance, a clearance between the friction plate and the separator plate has been made smaller than that those in the conventional clutches, with the result that, during an idle rotation, drag torque caused by an oil film existing between the friction plate, and the separator plate tends to be increased.
The oil supplied to the oil passage extending through from the inner diameter side to the outer diameter side is drawn into the friction material by the rotation, and, if the drawn oil enters between the friction plate and the separator plate, the oil is hard to be discharged. Particularly, in the case where the clearance between the friction plate and the separator plate is small, the difficulty of discharging of the oil becomes noticeable in an area where revolution per minute is small, thereby increasing the drag torque due to viscosity between the friction material and the opposing separator plate.
Although the oil passage provided to extend through between the inner diameter side and the outer diameter side serves to supply the oil to the friction surface and to discharge the oil, the configuration of the oil groove and/or both dull corners of the oil groove affect a great influence upon the flow of the oil from the oil groove to the friction surface, with the result that the idle rotation torque may be increased.
In the conventional friction plates, since the oil could not discharge adequately from the friction surface, the request for further reducing the drag torque could not be satisfied. Particularly, in the low rotational speed area, the drag torque could not be reduced.
However, in order to meet the requirements regarding compactness and light-weight sought by recent automatic transmissions, it is desired that a friction capacity of each friction plate be increased. To this end, increase in the number of grooves and dimension of the groove has been controlled severely.